This invention relates generally to tuning systems for television signals and specifically to an all-channel tuning system for tuning all over-the-air broadcast television signals and cable-connected television signals.
The broadcast television spectrum consists of 6 MHz wide channels in a plurality of disconnected frequency bands. These are commonly referred to as the VHF low and VHF high frequency bands, comprising channels 2 through 6 and 7 through 13 and covering 54 MHz through 88 MHz and 174 MHz through 260 MHz, respectively, and a UHF band covering channels 14 through 83 and extending from 470 MHz through 890 MHz.
The mechanisms for tuning these channels were initially large, cumbersome mechanical tuners that switched in different coil and capacitor combinations to attain the appropriate frequencies for television signal reception. A modern television tuner comprises a relatively simple electronic device having varactor diodes as the primary tuning control elements. The various coil arrangements used with the varactor diodes to tune the different frequency ranges are switched by so-called bandswitching diodes. As is well known, a varactor diode exhibits a capacitance which varies as a function of an applied DC tuning voltage. Since the amount of capacitance variation required to tune a circuit is primarily a function of the ratio between the highest and lowest frequencies involved, it is still necessary to bandswitch to embrace all of the so-called VHF frequencies. The UHF band, on the other hand, has generally been tunable with a single set of non-bandswitched tuned circuits.
The antenna structures used to receive VHF signals differ significantly from those used to receive UHF signals. In metropolitan areas, for example, UHF signals may be adequately received with a very small, low cost antenna structure, such as a "bow-tie" or a loop, whereas the VHF frequencies require a much larger structure, generally positioned outside at a high point such as on a rooftop. The bow-tie and loop are eminently suited to equally low cost 300 ohm impedance terminations, with the result that the UHF tuner has been universally supplied with a 300 ohm input impedance and as a separate structure entirely apart from the VHF tuner. Even with modern UHF/VHF electronic tuners, the historic 300 ohm UHF tuner section, like its forebear, has been retained as a separate structure.
In cable antenna television systems (CATV), cable operators are free from many of the FCC-imposed restrictions on television signal make-up. Since a cable-connected signal is not radiated into the air, use is made of the large frequency gap in the VHF signal spectrum between channels 6 and 7 and that existing between the upper end of the VHF spectrum, i.e., channel 13, and the lower end of the UHF spectrum, i.e., channel 14. For obvious reasons, cable operators generally selected frequencies compatible with existing receiver hardware. Television manufacturers, in turn, manufactured receivers for use with both air and cable signals. As the art developed, it became commonplace for VHF tuners to be used with converters for receiving cable-connected signals in much higher frequency bands and down-converting them to one of the VHF tuner channel positions, generally channel 3 or 4. Other developments occurred rapidly in which the VHF tuner ranges were extended to encompass the so-called CATV "midband", "superband" and "hyperband" frequencies. The upper limit on the hyperband has been steadily raised toward the lower end of the UHF spectrum. To avoid confusion it has been proposed to use the term "ultraband" for those CATV signals above the hyperband, which signals may encompass the UHF spectrum of frequencies.
Major advances have occurred with the use of microprocessor controlled tuning systems in television receivers. In such systems, tuning voltages and band signals for determining the appropriate set of tuned circuits, are stored in a memory that is generally accessed by the channel number. Systems of this type have been in use for a number of years and are well known in the art. While the channel selector means are significantly different, the tuning systems still incorporate a conventional 300 ohm UHF tuner for tuning signals in the UHF spectrum and either an additional VHF type tuner for tuning to CATV channels or an extended range VHF tuner which may have three or more different tuning bands for covering the VHF spectrum and the CATV midband, superband and hyperbands. A European television receiver has been marketed with a single 75 ohm input tuner with separate VHF and UHF sections of otherwise conventional design.
With the growing acceptance of CATV has come a commensurate need for additional CATV channels. UHF television stations, on the other hand, have suffered and have generally failed to develop markets equivalent to their VHF counterpart stations. Recently, FCC truncated the UHF spectrum beyond channel 69, thus removing about 80 MHz from the upper end of the UHF band. Despite this change, tuning systems continued to follow the prior art format of dual inputs, one for the VHF and low frequency CATV sections and one for the UHF section. The UHF tuner in the UHF section was, as it always has been, limited to tuning only broadcast UHF signals. The prior art solution to the growing need for additional CATV channel tuning capability failed to recognize the air and cable channel frequencies as a continuum and merely added additional bands of higher frequency VHF tuning or additional tuners for the CATV signals. The results were very expensive and complex mechanisms for "almost-all-channel" tuning. Thus the prior art failed to provide a tuning system for all 178 channels, both airborne and cable, and one which was sufficiently economical to be used in nearly all television receivers.